The genetic maps of the enteric bacteria Salmonella and E. coli have remained remarkably similar despite the fact that these organisms have been separated for substantial evolutionary time and their sequence homology has diverged. This suggests that selective forces have prevented survival of cells that have undergone chromosome rearrangement. Despite evolutionary stability of the genome, many events are known that cause rearrangement of the chromosome. These include transposable elements and the events they induce, and spontaneous duplication and inversion events that seem to occur by recombination between repeated sequences that are separated in the chromosome. We will investigate these events by studying IS sequences and drug resistance transposons, and by devising genetic means of selecting and studying rearrangements. We'll study the mechanisms of rearrangements and the physiological consequences of these rearrangements. In particular, we will pursue the idea that gene duplication is an advantageous event for bacteria. We suspect that the structure of the bacterial chromosome includes sequences located appropriately for generation of advantageous duplications.